Control system



1941' T. B. MONTGOMERY ETAL 2,251,493

CONTROL SYSTEM Filed Dec. 18, 1937 Patented Aug. 5, 1941 CONTROL SYSTEMTerryl B. Montgomery, Wauwatosa, and Edward H. Stivender, West Allis,Wia, assignors to Allis-Chalmers Manufacturing Company, Milwaukee, Wia,a corporation of Delaware Application December 18, 1937, Serial No.180,520

- 13 Claims.

This invention relates in general to electrical control systems andparticularly to electrical systems for controlling a strip of materialbeing wound upon and unwound from reels or similar devices while anoperation is being performed on the strip and for controlling andindicating the tension applied to such strip during such operation.

In rolling mills where steel strip is passed between rolls to change itsdimensions, outer surface or internal structure, it has been founddesirable to maintain the strip under tension during such rolling and tomaintain this tension as nearly constant as practicable. Tensioning maybe provided by connecting dynamo electric machines to the reels or torolls at various points along the strip and by controlling the power ofsuch machines. The power (input if a motor, output is a generator) ofany tensioning machine must be maintained constant for a constanttension at a given constant strip speed. If the strip speed is varied,the tensioning power must be varied to keep the tension constant. Theloss or gain in tensioning power due to acceleration and deceleration ofthe strip must be accounted for to maintain a constant tension. Thetension may be indicated by an electrical instrument reading thetensioning machine power orsome factor thereof suitably compensated.

It is therefore an object of this invention to provide an improvedcontrol system for a strip rolling mill wherein a more nearly constanttension can be simply, reliablyand automatically maintained.

It is a further object of this invention to provide in a striptensioning system a control providing substantially constant tension onthe strip It is a further object of this invention to provide a steelstrip rolling mill control system in which the tension control isenergized from a source of energy having a relatively great differenceof potential thereacross but a relatively low voltage therefrom toground.

Objects and advantages other than those above set forth will be apparentfrom the following description when read in connection with the ac-'companying drawing in which the single figure is a diagrammaticrepresentation of a strip rolling mill and control system thereforembodying the present invention.

A strip of material I is unwound from a reel 6, passed through the millrolls l3 and wound upon the reel 8 in the arrow direction shown. Themill rolls it have large backup rolls [4 and are driven by the main millmotor l5 energized from a generator it operated by a suitable source ofpower such as a synchronous motor H. In the system described, the strip5 is tensioned at three points by the dynamo electric machine 1connected mechanically to reel 6, the dynamo elec-- tric machine 8connected mechanically to the reel I and the dynamo electric machine I:connected mechanically to the strip 5 by one or both of the rolls Ii.The speed of the mill, and hence the speed of the strip through therolls I3, is controlled by controlling the field current in the fieldsII and it of the generator I6 and the motor I5.

during acceleration and deceleration of the strip.

It is a further object of this invention to pro vide in a striptensioning system a control providing substantially constant tension onthe strip regardless of change in strip speed.

It is a further objectof this invention to provide in a strip tensioningsystem a control providing substantially constant tension on the stripby an improved compensation for inertia effect of the mill duringchanges in strip speed.

It is a further object of this invention to pro,- vide in a controlsystem utilizing any or all of the above features to maintain constantstrip ten-.

sion, a simple and accurate means of indicating the strip tension.

It is a further object of this invention to provide in a striptensioning system of the above type, tension indicating meanscompensated for increased'length of strip due to reduction of thethickness thereof.

It is a further object of this invention to provide in a striptensioning system a means for adiusting the strip tension which will beaccurate at very low or zero tensions.

The speed control is effected by a motor operated rheostat showngenerally at 43 having a motor 52 provided with windings 53 and 54whereby it is rotatable in either direction to move the contact bar 49to the left or right as viewed in the drawing. Movement of the contactbar 9 to the left decreases and to the right increases the speed of themill. In the lower speed range, the speed change is eii'ected bydecreasing the amount of resistance 59 in circuit with the generatorfield it. In the upper speed range, the speed change is effected bydecreasing the amount of resistance in circuit with the motor field it.This permits starting the mill with full armature current and fulltorque on the motor ii.

The three tensioning machines I, 9 and [2 are I regulated to maintainthe tension on the strip substantially constant by means of regulators25, 28 and 21 respectively. The pull transferred by any of thesemachines to the strip 5 is equal to the power (input, if a motor andoutput, if a generator) divided by the speed of the strip. Hence for agiven strip speed, which is substantially constant for a given pass, thepower of the tensioning machines must be maintained constant in order tomaintain a constant tension.

The machies 'I and 9 are connected in series loop with a boostergenerator 28 which has a reg ulator 32, responsive to the current ininterpole winding 33, for controlling the current in the field 29 sothat a constant current flows in the loop circuit. A rheostat 81 isprovided for adjusting the value at which the current is maintainedconstant by the regulator 32. With a constant current flowing in thearmatures of machines 1 and 9, the voltage across each machine ismaintained constant in order to maintain a constant power (input oroutput) and therefore a constant tension supplied to the strip. Thevoltage across the machine 9, for example, is maintained constant by theregulator 26 which controls the current through the field 24 in responseto energization of coil 45 responsive to the voltage across machine 9.Thus the power supplied motor 9 remains constant regardless of speedchanges due to the building up of material of the reel 8. The core uponwhich the coil 45 acts is biased downwardly in any suitable manner, forexample, as by gravity, and the coil 45 tends to pull upwardly on thecore. For a given strip speed and tension the current in coil 45 isconstant and the regulator is balanced.

If the speed of the strip 5 is maintained constant and a constanttension thereon is maintained, the speed of the machine 9 mustnecessarily decrease as the material builds up on the reel 8. Toconsider the above change, let us note the eiiect of a small change inbuild up of material on the reel.

First, a small increas of material, increases proportionally the leverarm through which the motor torque acts on the strip. Assuming the stripspeed constant, it would be necessary to increase the power supplied toth motor 9 in proportion to the lever arm increase in order to maintainthe motor speed constant. However, with constant strip speed, as nochange in power supplied the motor has yet been effected, the motorspeed decreases proportionally to the lever arm increase. The decreasein speed of motor 9 decreases the generated voltage thereof, therebydecreasing silghtly the power supplied to the motor and consequentlyslightly decreasing the tension in strip 5. To return the power (andthereby the strip tension) to normal, the decrease in voltage across themotor 9 decreases the current flowing in the coil 45 and allows the coreto move downwardly'due to the bias thereon.

The downward core movement decreases the regulator resistance, therebyincreasing the current through the field 24 which in turn increases thepower supplied to the motor to its normal value and the regulator 26 isagain balanced and the tension is again nomal. The regulator regulatesthe generator 7 in a similar manner by controlling the field 23.

In order to hold a constant tension on the tensioning motor l2, it isnecessary to supply a constant power to this motor for a given speed ofstrip 5. This power must be increased or decreased directly as the stripspeed increases or decreases to maintain tension constant. Therefore itis evident that for a given strip speed and tension, the voltage acrossmotor I2 is constant and if a constant current is maintained in thearmature thereof, the tensioning mot'or power and therefore the pull onthe strip will be constant. The main coil 55 of the regulator 27 istherefore connected across the interpole winding 34 of the motor (2 andoperates the regulator to vary the current in the field 22 of the motorl2 so that the current through the interpole winding 34 (and thereforemotor current) is maintained constant.

The above provisions for maintaining a constant tension on the stripapply only for a given constant strip speed. When the speed of the millis changed, the tensioning control means must be compensated therefor inorder to maintain the tension constant. This compensation is provided byresistances 63, 62 and 64 in series with the coils 45, 35 and 55respectively. Increments of these resistances are added to theirrespective circuits upon movement of the bar 49 to the right, as shownin the drawing, which increases the speed of the mill.

For example, increasing the resistance 53 in series with the coil ofregulator 26 causes an increase in the voltage across the motor 9 inorder to balance the regulator. The resistance 53 is calibrated so thatthe increase in motor voltage (and hence motor power as the motorcurrent is constant) is proportional to the strip speed change. Thetension is therefore automatically maintained constant regardless ofchange of speed of the strip 5.- The changes in resistance 62 similarlycompensate the control of generator 1.

As previously stated, the voltage of the generator I6 is constant onlyover the upper speed range of the mill During the lower range of millspeed the increase in speed of the mill, and therefore the speed of thestrip, is directly proportional to the generator voltage. The generatorvoltage is also the voltage across the machine l2. Therefore, during thelower range of speed, the tension is maintained constant by maintaininga constant current through the motor l2 and by increasing the voltageacross the motor l2 directly proportional to the speed of the strip.

During the upper speed range the voltage of generator 16 and hencevoltage of motor I2 is maintained constant. Upon raising speed in thisrange, added increments of resistance 64 are included in series withcoil of regulator 21, necessitating a greater voltage drop across theinterpoles 34 to balance the regulator. The resistance 64 is calibratedso that the regulator 21 increases the power supplied motor l2 directlyas the speed changes and the tension is therefore maintained constant.

In order that the regulator 25 be responsive to that part of theterminal voltage of the machine 9 available for power transmitted to thereel 8, the current in coil 45 is compensated by a voltage drop througha resistor 72. This resistor l2 is-energized from an outside source andthe voltage drop there across is made equal to the IR drop through thearmature of motor 9. In the mill shown resistor 12 could be suitablyconnected across the excitation circuits 85, 86. however, if a similarcompensation is to be also provided for the machine 1, a separateoutside source such as the rectifier and alternating current sourceshown must be used in order that the circuits may be separate.

This IR. drop compensation can generally be omitted from the draggenerator regulator 25, as the entry tension is seldom sufficiently lowin value to make such compensation necessary. The eiTect of the 1R dropbecomes negligible at high field strengths. The delivery reel motor 9may frequently, in practice, run at nearly zero torque.

Provision is made for adjusting the tension in a new and improvedmanner. Separate adjusting coils 31, 41 and 51 are provided on theregulators 25, 26 and 2'! and the current in these adjusting coils maybe varied manually by the rheostats 38, 48 and 58. The utilization ofseparate adjusting coils provides definite advantages. The regulatorscan operate effectively and accurately at low and even zero current inthe main coils. Indication of tension can be effected in an improvedmanner.

The regulators 25, and 21 each always balances against the biasing forceat a fixed number of ampere turns in the solenoid. These ampere turnsmay be supplied by one or more of the regulator coils, but the totalampere turns supplied is constant. The ampere turns of the adjustingcoils 31, 41 and 51 add to those of coils 35, 45 and 55.

For example, suppose the rheostat 48 is in its extreme left position sothat all the resistance is in circuit with coil 41, and that in suchposition the coil 45 takes two amperes to balance the regulator. It isthen desired to decrease the tension and the rheostat 48 is moved to theright cutting out half of its resistance and thereby adding ampere turnsto coil 41 in such amount that only one-half the ampere turns in themain coil 45 are now required to balance the regulator. This means thatonly one ampere is required in coil 45. As the resistance of main coilcircuit does not change, one ampere has a voltage drop one-half that oftwo amperes. Thus by moving the rheostat 48 one-halt its distance, thevoltage of motor 8 decreases to one-half its value and as the motorcurrent is constant, the power and therefore tension decreases toonehalf its value. The fact that the change in regulator coil current isdirectly proportional to 1 tension change is utilized in an improvedtension indicating means.

In prior art control, the tension is adjusted by change of a resistancein series with the main coil of a vibratory regulator, the tension beingdecreased by decreasing the amount of resistance in series. In suchsystems, the tension could not be decreased to zero or low tensions dueto the fact that the regulator coil itself had resistance which couldnot be cutout. The regulators utilized in the prior art were of thevibrating type which at zero and low current would become inoperative.

In the case of the tension regulator 21 this type of adjusting coilprovides further advantages. This rheostat is manually operated tochange the resistance in series with the main regulator coil and ishandled by the mill operator. The prior art rheostats are energized fromthe relatively high voltage circuit of the main power circuit. The coil51 is energized from the relatively low voltage excitation circuit 85,85 and hence there is less danger to a person operating a rheostat 58.In the prior art control the adlusting resistance operates on arelatively low difference of potential or voltage drop across suchresistances. Coil 51 operates on a relatively high difference ofpotential or voltage drop thereacross and therefore a more accurate,smootheradjustment can be obtained in that a greater number of stepswith a greater difference in voltage for each step is provided. Coarseand Vernier rheostats are not required. Also with the high drop rheostat58, the leads and rheostat contacts are effected less easily by greaseand dirt. This is important in steel mill control systems where themanually operated rheostats such as 58 must necessarily be near to themill and therefore sub- The use of the high jected to the dirt thereof.

drop rheostat permits it to be made smaller in size.

Other important advantages are derived from connecting the coil 55 ofthe balance regulator 21 across the interpole winding 34 of thetensioning machine. The interpole winding 34 serves as an inductiveshunt in contradistinction to the non-inductive shunts utilized in theprior art to energize a vibratory type of regulator. By using thebalanced type of regulator, a low voltage drop such as is obtained fromthe interpole winding 34 may be used. The vibratory type of regulatorneeds a greater voltage drop for energization thereof. In the inductiveshunt as shown, the regulator is not only operated by the change incurrent through the shunt, but also receives the inductive kick due tothe voltage induced by the current change in the shunt. Another featureof the use of this shunt is that it is made entirely of copper andtherefore as the regulator coil is entirely of copper there is no changein the relative balance due to change in the temperature. In the priorart system where a vibratory type of regulator is used it is necessaryto use iron grids to obtain sufiicient voltage drop to operate theregulator. Temperature changes of the iron cause a change in current andtherefore destroy the accuracy of the regulator operation.

Upon acceleration or deceleration of the mill, the power necessary forthe acceleration or deceleration must be added or subtracted as the casemay be. This is accomplished by the coils 35, 45 and 55 of the regulatorwhich are connected in series circuit and are energized with current ofone polarity or the other through contacts 51 or 59. Upon accelerationpower must be added to tensioning machines on the delivery end of thestrip. When the speed change switch 58 is closed to raise the speed ofthe mill, contact 58 closes a circuit from the positive lead through thecoils 55, 45 and 35 through the resistor 84 to the negative lead 55.Coil 55 is therefore energized to subtract ampere turns from theregulator so that more power will be added to the motor I! to balancethe regulator 21.

On the entry end of the mill, machine 1 being a generator, uponacceleration of the mill, coil 35 will add ampere turns to the regulator25 so that less power in the machine 1 will balance the regulator. Asimilar action in a reverse direction takes place when the switch 55 isclosed to lower the speed of the mill. The contactor 51 closes a circuitfrom the negative conductor 85 through the coils 55, 46 and 35, throughresistor 83 to the positive conductor 85. This reverses the energizationof these coils relative to the energization upon acceleration therebytaking away energy from the machine l2 or 9 on the delivery end of themill and adding energy to the machine 1 on the entry end of the mill.

This inertia (WR compensation, utilizing separate coils on the regulatorsolenoid actuated by the speed change initiating means, is animprovement over prior art systems utilizing means responsive to therate of speed change of the mill. The prior art systems could not supplythe power change quickly enough to keep the tension in the strip. First,the acceleration compensation was initiated only after the speed changedand then was further belated due to the time constant of the motor fieldto be compensated The present system of WR compensation starts thecompenastion control prior to the speed change, so that the compensationand speed change are simultaneous and the compensation thereforeefiective to maintain constant tension. Due to the fact that therheostat motor 52 runs at a constant speed the rate of acceleration ordeceleration is always the same and the need for added apparatus toprovide WE, compensation in accordance with the rate of speed change isavoided.

The present invention also includes an improved means for indicating thetension applied to the strip. In order that such indication may beaccurate, compensation must be effected for changes of speed of themill. Tension indicators 16 and 18 are provided for the machines I and8, respectively, and these indicators each have a coil energized from ashunt 82 in series with the machines I and 9. These indicators are ofthe wattmeter type and each have a potential coil connected across asource of current proportional to the current in the regulator maincoil.

As previously stated the current in the main regulator coil 56 isproportional to the tension supplied by motor 22 and the current in themain regulator coils 35 and 45 are proportional to the tension ofmachines 1. and 8 if the current in the loop circuit is maintainedconstant. The potential coil of indicator i8 is connected across aresistance '19 in the circuit of coil 45 and therefore may be calibrateddirectly to read pounds tension.

The potential coil of indicator '18 is energized from a sourceartificially made to represent current in regulator coil 35. This isdone by conmeeting the indicator coil across a potentiometer rheostat Tienergized from the constant potential source 8t 36. The rheostat Ti isin tandem with the tension adjusting rheostat 3B, the movement oi whichis proportional to the current in regulator coil 35 as previouslyexplained. The current in the potential coils of the indicators istherefore always proportional to the regulator coil current.

cases where tension on the loop circuit machines and 9 is desired abovethat provided by maximum field strength, this may be supplied byincreasing the loop circuit current by changing the setting or"regulator 32 of the booster generator 28. The changed current throughthe current coils of the indicators connected to shunt 82 provides thatthese indicators properly indicate the tension.

The use of the rheostat 77 on the entry end of the mill permits animproved compensation of the tension adjustment and indication due toreduction of the strip (increase in length) after passing through therolls. This compensation is provided by the rheostat 39, connected inparallel with rheostat 38, and provided with a scale graduated inpercent reduction. This rheostat is manually set for the reduction of agiven mill pass and shunts rheostat 38 to a given degree so that itsposition and that of rheostat l"! are proper for the tension.

For example, if the reduction is fifty percent, machine i must operateat only one-half the speed of machine 9 to supply the same tension. Bymeans of rheostat 39, rheostat 38 provides ampere turns in coil 3? forone-half voltage while rheostat 38 is in full voltage position, therebyproperly operating rheostat T! to indicate truly and accurately thetension. If the mill is a reversing mill, the compensation of the entrytension indicator must be provided at both ends. If the mill is of thetype in which little or no reduction is taken, such as a temper millthrough which the steel strip is passing merely to obtain better grainstructure or surface, indicators of the type of 18 may be used on bothends of the mill.

The ammeter H is connected so as to indicate directly the tensionprovided by the tensioning motor l2. This indicator reads the current inthe regulator coil 55 and as previously explained this current is anaccurate indication of tension.

This is an improvement over prior art systems of control which need awattmeter type instrument to indicate tension.

Although but one embodiment of the present invention has beenillustrated and described, it will be apparent to those skilled in theart that various changes and modifications may be made therein withoutdeparting from the spirit of the invention or from the scope of theappended claims.

It is claimed and desired to secure by Letters Patent:

1. In a control system for winding and reeling a strip of material, adynamo-electric ma.- chine mechanically connected to said strip fortensioning the same, means for maintaining said tension substantiallyconstant comprising an electroresponsive device provided with a firstwinding operatively responsive to a change in a characteristic of saiddynamo-electric machine. and means for adjusting said tension to achanged substantially constant value comprising a second winding on saiddevice operative to maintain the current in said first windingproportional to said tension.

2. In a control system for winding and reeling a strip of material, adynamo-electric machine mechanically connected to said strip fortensioning the same, means for maintaining said tension substantiallyconstant comprising an electroresponsive device provided with a firstwinding operatively responsive to a change in a characteristic of saiddynamo-electric machine, means for performing an operation on saidstrip, means for varying the speed of said operation performing means,means connected in circuit with said first winding for indicating thetension on said strip, and means responsive to operation of said speedvarying means for varying the energization of said first winding andsaid indicating means.

3. In a control system for Winding and reeling a strip of material, adynamo-electric machine mechanically connected to said strip fortensioning the same, means for maintaining said tension substantiallyconstant comprising an electroresponsive device provided with a firstwinding operatively responsive to a change in a characteristic of saiddynamo-electric machine variable with a change in said tension, meansfor varying said tension, means connected in circuit with said Windingfor indicating said tension, and means operatively responsive tooperation of said tension varying means for compensating said-tensionindicating means, so as to correctly indicate the changed tension.

4. In a control system for winding and reeling a strip of material, adynamo-electric machine mechanically connected to said strip fortensioning the same, means for maintaining said tension substantiallyconstant comprising an electroresponsive device provided with a firstwinding operatively responsive to a change in a character istic of saiddynamo-electric machine variable with a change in said tension, meansfor varying said tension, means connected in circuit with 2,251,4iil8said winding for indicating said tension, means for performing areducing operation on said strip, means operatively responsive tooperation of said tension varying means for compensating said tensionindicating means for said tension change, and means for modifying saidtension varying means to compensate for reduction of said strip.

5. In a control system for winding and reeling a strip of material, adynamo-electric machine mechanically connected to said strip fortensioning the same, means for maintaining said ten sion substantiallyconstant comprising an electroresponsive device provided with a firstwinding operatively responsive to a change in a characteristic of saiddynamo-electric machine, means for performing an operation on saidstrip, means for varying the speed of said operation performing means,and means responsive to initiation of operation of said speed varyingmeans for compensating said first winding for said speed change,

6. In a control system for winding and reeling a strip of material, adynamo-electric machine mechanically connected to said strip fortensioning the same, means for maintaining said tension substantiallyconstant comprising an eiectroresponsive device provided with a firstwind.-

ing operatively responsive to a change in a chai acteristic of saiddynamo-electric machine, means for adjusting said tension to a changedsubstantially constant value comprising a second winding on said deviceoperatively independ ent of said changes in said characteristic of saiddynamo-electric machine, and tension indicating means energizedproportionally to the current in.

said first winding.

7. In a control system for winding and reeling a strip of material, adynamo-electric machine mechanically connected to said strip fortensioning the same, means for maintaining said tensionsubstantiallyconstant comprising an electrore" sponsive device providedwith a first winding operatively responsive to a change in a oharacteristic of said dynamo-electric machine, means for performing anoperation on said strip, a motor mechanically connected to saidoperation performing means and electrically connected in circuit withsaid dynamo-electric machine, means for adjusting said tension to achanged substan tially constant value comprising a second winding onsaid device, and means for energizing second winding independently ofsaid motor cir" cuit.

8. In combination, a dynamo-electric machine, means including anelectroresponsive device provided with a first winding operativelyresponsive to a change in a' controlling characteristic of saiddynamo-electric machine for maintaining a corn trolled characteristicthereof substantially con stant, and means for adjusting said controlledcharacteristic to a changed substantially constant value comprising asecond winding on said device operative to maintain the current in saidfirst,

winding proportional to said controlled characteristio.

9. In a tension controlling system for winding and reeling a strip ofmaterial, a dynamo-electric machine mechanically connected to saidstrip, a balanced regulator for controlling the power of dynamo-electricmachine comprising an eiectroresponsive device provided with a firstwinding connected across the terminals of said dynamo-electric machine,and means for adjusting said tension comprising a second winding on saiddevice energized independently of said armature circuit,

iii. in tension controlling system for winding and reeling a strip ofmaterial, a dynamoelectric machine mechanically connected to said strip,a balanced regulator for controlling the power of said dynamo electricmachine comprising an electroresponsive device provided with a firstwinding operatively responsive to the voltage across saiddynamo-electric machine, a secand winding on said electroresponsivedevice, and means energized from a constant voltage source for adding orsubtracting ampere turns to said second winding.

11., In a tension controlling system for win ing and reeling a strip oi:material, a dynamm electric machine mechanically connected to saidstrip, a balanced regulator for controlling the power of saiddynamo-electric machine compris ing an electr: esponsive device providedwith a first winding connected across the terminals of. saiddynamo-electric machine, and means for compensating the energization ofsaid first winch lug whereby it is responsive to the generated voltageof said dynamo-electric machine.

12. In a control system for a strip of material adapted to be unwoundfrom a first reel and wound upon second reel, dynamo-electric machineconnected to said first reel, 9, second dynamo-electric machineconnected to said sec-- ond reel, means connecting the armatures of saiddynamo-electric machines series, means for performing an operation onsaid strip during said unwinding and windin motor i'or driving saidoperation peiiorining i cans, means for varying the speed oi said motor,a second motor meohanically connected. to said strip between saidoperation performing means and one or said dynan1oelectric mac lies,means connecting the armaturcs oi" said motors in parallel, means forregulating the power supplied said second motor comprising iirstelectroresponsive device having a va'inding, means for regulating thepower of one of said dynammelectric machines comprising a secondelectroresponsive device having a wind" tension indicator comprisiiresponsive the current in inding oi said first electroresponsive device,and a tension indicator con lsing wattmeter provided with t h'ao ,siveto current proportional to the current .n said winding of said secondclcctroresponsive device.

13. control system for Winding and reeling a strip of r erisl, adynammelectric machine connected to said reel, means for regal" 1 poweroi said dynamo-electric machine coinprising an elcctroresponsive deviceprovided with Winding, and means opei'ativcly responsive to the armaturecurrent of said dynamo-"electric mact'ne and to a current proportionalto the out .t in said winding for indicating the tensi 1 supplied saidstrip by said dynamo-electric machine.

